This invention relates to fuel cells and, in particular, to detection of sulfur breakthrough in a desulfurizer assembly used with such fuel cells.
A fuel cell is a device which directly converts chemical energy stored in hydrocarbon fuel into electrical energy by means of an electrochemical reaction. Generally, a fuel cell comprises an anode and a cathode separated by an electrolyte, which serves to conduct electrically charged ions. Molten carbonate fuel cells operate by passing a reactant fuel gas through the anode, while oxidizing gas is passed through the cathode. In order to produce a useful power level, a number of individual fuel cells are stacked in series with an electrically conductive separator plate between each cell.
Current fuel cell technology requires clean fuel gas composed of hydrogen or a mixture of hydrogen and carbon monoxide, which can be generated from hydrocarbon-containing feedstocks such as natural gas, propane, anaerobic digester gas, petroleum-based liquids or coal through a reforming process. Most hydrocarbon-containing feedstocks contain sulfur, which causes reforming and anode catalyst poisoning and is known to significantly diminish the performance of fuel cell anodes and reforming catalysts. Therefore, prior to the reforming process, sulfur and sulfur-containing compounds have to be removed from the fuel gas to a part per billion level before the fuel gas enters the fuel cell.
The present state of the art employs a fuel processing assembly, such as a desulfurizer assembly, that includes at least one adsorption bed for removal of sulfur-containing compounds from the fuel gas before passing the fuel gas to the fuel cell anode. An example of such a fuel processing assembly is disclosed in U.S. Pat. No. 7,063,732, which is assigned to the same assignee herein. In particular, the '732 patent discloses a fuel processing system for processing fuel for a fuel cell including a first adsorbent bed for adsorption of inorganic sulfur-containing compounds and high molecular weight organic sulfur-containing compounds and a second adsorbent bed for adsorption of low molecular weight organic sulfur-containing compounds, wherein the adsorbent beds are arranged such that the fuel to be processed passes through one of the adsorbent beds and thereafter through the other of the adsorbent beds.
As can be appreciated, the adsorbent capacity and performance of the adsorbent bed used in the fuel processing system declines with operating time as the adsorbent bed becomes more saturated with sulfur-containing compounds. As a result, sulfur breakthrough occurs when the adsorbent bed becomes unable to decrease the concentration of the sulfur-containing compounds in the fuel to a desired level, normally expressed in parts per billion by volume (ppbv), and the amount of sulfur-containing compounds passing through the bed without being adsorbed, i.e. sulfur breakthrough concentration, increases as the saturation level of the sulfur-containing compounds in the bed is achieved. When a predetermined sulfur breakthrough concentration in the processed fuel is reached, the adsorbent bed has to be replaced or regenerated to avoid sulfur poisoning of the fuel cell system components. Due to variable concentrations of sulfur-containing compounds in the fuel gas, the time when the predetermined level of sulfur breakthrough is reached can be highly variable. Therefore in order to ensure timely replacement or regeneration of the adsorbent bed, monitoring of the sulfur breakthrough concentration in the processed fuel is required.
Presently, the monitoring of the sulfur breakthrough concentration is accomplished by intermittently analyzing samples of processed fuel gas leaving the fuel processing assembly using conventional gas chromatography techniques. Commonly used techniques for analyzing the sulfur concentration in the processed fuel include Gas Chromatography (GC) in conjunction with Sulfur Chemiluminescence Detection (GC-SCD) or Flame Photometric Detection (GC-FPD) techniques. However, these conventional techniques are expensive, thus substantially increasing the fuel processing costs and the operating costs of the fuel cell system. Moreover, the conventional monitoring methods require sampling of the processed fuel and therefore, require additional personnel and additional analytical equipment for sample collection, transportation from the field to a laboratory and performing the analysis of the processed fuel samples. As a result, the conventional methods cannot be integrated with the fuel cell processing assembly so as to continuously monitor the breakthrough sulfur concentration online.
It is therefore an object of the invention to provide a sulfur breakthrough monitoring assembly and method for use with the fuel processing system which is able to continuously monitor for sulfur breakthrough concentration on-line.
It is a further object of the invention to provide a sulfur breakthrough monitoring assembly and method which is integrated with the fuel processing system so as to continuously monitor for sulfur breakthrough concentration online, without requirement of sampling the processed fuel for analysis by outside analytical equipment.
It is a further object of the invention to provide a sulfur breakthrough monitoring assembly and method which is highly accurate in detection of sulfur breakthrough and is cost effective.